Cemented metal carbides and other cermets, polycrystalline diamond (PCD), and cubic boron nitride (CBN), and combinations of them, have been used for many years for cutting tools, hard facing, wear inserts, cutting inserts, and other wear parts and surfaces in various types of tools because of their desirable properties of hardness, toughness and wear resistance. Cemented metal carbide refers to a carbide of one of the group IVB, VB, or VIB metals which is pressed and sintered in the presence of a binder of cobalt, nickel, or iron and the alloys thereof. The most common example of a cemented metal carbide used in downhole applications is tungsten carbide (WC). Polycrystalline diamond is made by sintering powdered diamond in the presence of a catalyst, such as a cobalt alloy or nickel, resulting in intercystalline bonding between individual diamond crystals. The diamond can be synthetic or natural diamond, cubic boron nitride, or wurtzite boron nitride as well as combinations thereof. PCD is typically utilized in wear applications as a crown layer attached to a base comprised of cemented WC. Such an insert is sometimes referred to as a polycrystalline diamond compact (PDC).
Drill bits, rock mills and other earth boring tools used in oil and gas exploration are examples of tools that make use of wear resistant inserts for surfaces that will be subject to substantial abrasion and wear. Examples of inserts with abrasion resistant wear surfaces include abrasive jet nozzles, long life wear parts, carbide cutting tools, carbide wire drawing dies, cold heading dies, valve components (including seats), scuff plates, saw blades, deflector plates, milling tools, finishing tools, and various types of components for down hole tools, such as cutters and other inserts for earth boring bits (including rotary and drag bits) and bearing wear surfaces, such as mud-lubricated radial bearings and thrust bearings. An example of diamond bearing comprising a composite having a crown formed of PCD on a substrate of cemented carbide is as described by U.S. Pat. No. 4,729,440. Examples of cutters, bearings, and other types of inserts made from cemented metal carbides and PCD, and methods of manufacturing them, can be found in U.S. Pat. Nos. 6,500,226; 6,315,066; 6,126,895; 6,066,290; 6,063,333; 6,011,248; 6,004,505; 5,848,348; 5,816,347.
Inserts made from cemented metal carbide, PCD and cermets are joined to other components of a tool by either press fitting or brazing the insert. Brazing involves melting between two work pieces a filler metal having a melting point below the melting point of each of the work pieces, thereby forming a bond between the two work pieces. Examples of filler metal used for brazing are various alloys of cobalt. Brazing does not cause melting of either of the work pieces. Welding, on the other hand, requires heating adjacent portions of two work pieces above their respective melting points to form a pool of molten material, called weld pool, resulting in material from each piece inter-diffusing to form a bond that joins the pieces when cooled. Welding can be done either with or without the presence of a filler material.
Generally speaking, welding cemented metal carbide is not feasible or recommended due to stresses caused by heating of the cemented metal carbide. Although cemented metal carbides are very hard, tough, and resistant to wear, they are also relatively brittle. A small amount of strain can lead to its fracture. Furthermore, the more wear resistant, or harder, cemented tungsten carbide is made, the less tough and resistant to fracture it is. Uneven heating of a cemented metal carbide part leads to large temperature gradients across the part, which induces substantial stress across the part due to different degrees of thermal expansion caused by the uneven heating. Additionally, metal carbides also have a substantially different coefficient of thermal expansion as compared to stainless steel, which is the type of metal of which the bodies of and moving parts of down hole tools are fabricated due to is corrosion-resistance, strength and machinability. Heating a cemented metal carbide part and a steel part hot enough to melt the respective materials at the boundary between the two pieces creates substantial stress on the cemented metal carbide part when it cools. The stress typically leads to fracturing of the cemented metal carbide part during welding. If it does not immediately fracture, the residual stress within the part leads to substantially heightened susceptibility to fracturing when loaded, making the part not feasible for use, especially on downhole tools likely to experience high impact loads.